1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile.
2. Description of the Related Art
As an existing image forming apparatus, there has been proposed an apparatus using a developing device of a cleaner-less system, which removes a cleaning device in order to downsize the apparatus, and performs cleaning of a photosensitive drum (image bearing member) by a developing sleeve (developer bearing member) at the same time as developing (Japanese Patent Laid-Open No. 2001-215798, and Japanese Patent Laid-Open No. 2001-215799).
FIG. 10 is a configuration diagram of the existing developing device of the cleaner-less system. As in FIG. 10, in the developing device of the cleaner-less system, a counter developing method is generally used in which a direction of rotation Ra of a developing sleeve 4b and a direction of rotation Rb of a photosensitive drum 1 are opposite at an opposing portion (developing area) of the developing sleeve 4b and the photosensitive drum 1. Accordingly, at the opposing portion, two-component developer carried on the developing sleeve 4b physically scrapes off a transfer residual toner on the photosensitive drum 1, and increases recovery of the transfer residual toner.
In a developing device 4 of the counter developing method, however, the developer carried on the developing sleeve 4b receives a force in the opposite direction of a conveying direction of the developing sleeve 4b at the opposing portion. When this force in the opposite direction becomes large, the developer fails to be conveyed in the conveying direction of the developing sleeve 4b, whereby the developer on the developing sleeve 4b is retained near the opposing portion. Once the retention occurs, it may lead to an image defect such as adhesion of carrier and contamination inside the body of the image forming apparatus caused by the developer overflowed from the developing device 4.
One of such cases in which the retention actually occurs is when an amount of the developer carried on the developing sleeve 4b (hereinafter, referred to as M/S) exceeds an appropriate range relative to a set value of a closest distance between the developing sleeve 4b and the photosensitive drum 1 (hereinafter, referred to as S-D gap).
Such a situation often occurs when the developing sleeve 4b is driven at a lower speed. For example, transfer materials such as thick paper (basis weight of 150 g/m2 or above) and an OHT has a larger heat capacity than plain paper (basis weight from 60 g/m2 to 130 g/m2), whereby fixability in fixing a toner to the transfer material is inferior. Therefore, when performing image forming on these transfer materials, a process speed of the image forming apparatus should be set at a lower speed than that for the plain paper.
Furthermore, even though a developer M/S is within an appropriate range when the developing sleeve 4b is driven at a normal speed, the developer M/S may exceed an upper limit of the appropriate range when the developing sleeve 4b is driven at a lower speed.
Furthermore, a position of a magnetic pole of a magnet roller 4c fixedly disposed in the developing sleeve 4b at the opposing portion also affects the retention of the developer. The developer carried on the developing sleeve 4b forms a magnetic brush along a line of magnetic force formed by the magnet roller 4c on a surface of the developing sleeve 4b. 
Generally, a peak of magnetic flux density (developing pole) of the magnet roller 4c is often disposed in the vicinity of the opposing portion. At this time, just above or near the developing pole, the line of magnetic force extends in a vertical direction from the surface of the developing sleeve 4b. Then, as a distance from the developing pole becomes larger, the line of magnetic force formed on the surface of the developing sleeve 4b gradually comes down to a tangential direction of the developing sleeve 4b. Therefore, the force received by the developer in the opposite direction of the conveying direction at the opposing portion, or a force that hinders movement of the developer, is the largest just above the developing pole, and becomes weaker with distance from the developing pole.
The retention is more likely to occur when the force received by the developer in the opposite direction of the conveying direction at the opposing portion is large. Occurrence of the retention may be restrained by shifting the developing pole to a downstream or upstream side of a closest position of the developing sleeve 4b and the photosensitive drum 1. In Japanese Patent No. 4065481, the occurrence of the retention is restrained by shifting the developing pole 15 degrees to the downstream side of the closest position of the developing sleeve 4b and the photosensitive drum 1 at the opposing portion.
However, the technique in Japanese Patent No. 4065481 has a risk of decreasing image quality of an output image.
Just above and near the developing pole, the magnetic brush on the surface of the developing sleeve 4b is standing along the line of magnetic force, and an amount of developer per unit volume on the surface of the developing sleeve 4b is non-dense. Therefore, when a developing bias is applied, the toner existing in between a tip of the magnetic brush and near the surface of the developing sleeve 4b flies to the side of photosensitive drum 1, whereby a moving efficiency of the toner is high.
On the other hand, the magnetic brush on the surface of the developing sleeve 4b at a position away from the developing pole is lying down on the surface of the developing sleeve 4b along the line of magnetic force, and the amount of developer per unit volume on the surface of the developing sleeve 4b is dense. Therefore, it is difficult for the toner near the surface of the developing sleeve 4b to fly to the side of photosensitive drum 1, whereby the moving efficiency of the toner is low.
Therefore, in a case where the developing pole is shifted as in Japanese Patent No. 4065481, the moving efficiency of the toner at the opposing portion is low. Such a phenomenon noticeably appears when the developer deteriorates due to durability and the toner parting properties with the carrier declines, whereby an image defect such as insufficient density of a solid image or poor coarseness occurs.
Therefore, it is desirable for an image forming apparatus of the counter developing method according to an embodiment of the present invention to provide the image forming apparatus capable of stably forming a high-quality image without causing the retention in the vicinity of the opposing portion of the developer bearing member and the image bearing member, even in a case where the image forming is performed by driving the developer bearing member at a lower speed than in the normal speed.